1. Field
The present invention relates generally to electronic circuits, and more specifically to a direct downconversion receiver architecture for use in a wireless (e.g., CDMA) communication system.
2. Background
In a CDMA system, data to be transmitted is initially processed to generate a radio frequency (RF) modulated signal that is more suitable for transmission over a wireless communication channel. The RF modulated signal is then transmitted over the communication channel to one or more intended receivers, which may be terminals in the CDMA system. The transmitted signal is affected by various transmission phenomena, such as fading and multipath. These phenomena result in the RF modulated signal being received at the terminals at a wide range of signal power levels, which may be 100 dB or more.
At a given terminal, the transmitted signal is received, conditioned, and downconverted to baseband by a receiver front-end unit. Conventionally, the frequency downconversion from RF to baseband is performed with a heterodyne receiver that includes multiple (e.g., two) frequency downconversion stages. In the first stage, the received signal is downconverted from RF to an intermediate frequency (IF) where filtering and amplification are typically performed. And in the second stage, the IF signal is then downconverted from IF to baseband where additional processing is typically performed to recover the transmitted data.
The heterodyne receiver architecture provides several advantages. First, the IF frequency may be selected such that undesired inter-modulation (IM) products, which result from non-linearity in the RF and analog circuitry used to condition and downconvert the received signal, may be more easily filtered. Second, multiple filters and variable gain amplifier (VGA) stages may be provided at RF and IF to provide the necessary filtering and amplification for the received signal. For example, an RF amplifier may be designed to provide 40 dB of gain range and an IF amplifier may be designed to provide 60 dB of gain range, which would then collectively cover the 100 dB of dynamic range for the received signal.
For certain applications, such as cellular telephone, it is highly desirable to simplify the receiver design to reduce size and cost. Moreover, for mobile applications such as cellular telephone, it is highly desirable to reduce power consumption to extend battery life between recharges. For these applications, a direct downconversion receiver (which is also known as a homodyne receiver or a zero-IF receiver) may provide these desired benefits because it uses only one stage to directly downconvert the received signal from RF to baseband.
Several challenges are encountered in the design of a direct downconversion receiver. For example, because there is no IF signal in the direct downconversion receiver, the (e.g., 60 dB) gain range normally provided by the IF amplifier in the heterodyne receiver would need to be provided instead at either RF or baseband in the direct downconversion receiver. To avoid placing additional requirements on the RF circuitry and to reduce cost and circuit complexity, this IF gain range may be provided at baseband. However, if the baseband gain range is provided digitally after analog-to-digital conversion, then the baseband signal provided to the analog-to-digital converter (ADC) would have smaller amplitude since the gain is provided digitally after the ADC. DC offset in the baseband signal would then become a more critical consideration in the direct downconversion receiver because the baseband signal amplitude is smaller, and the DC offset may be a much larger percentage of the signal amplitude.
There is therefore a need in the art for a direct downconversion receiver architecture capable of providing the required signal gain and DC offset correction.